1. Technical Field
The present invention relates, generally, to semiconductor devices and, more particularly, to an improved heterojunction bipolar transistor incorporating a compound collector structure.
2. Background Information
In contrast to a standard homojunction transistor, a heterojunction bipolar transistor (HBT) includes at least two dissimilar semiconductor materials. That is, referring now to FIGS. 1A and 1B, while the emitter 102, base 104, and collector 106 of a standard homojunction transistor are formed from the same semiconductor material (e.g., Si or GaAs), in a single heterojunction bipolar transistor (SHBT), the emitter 108 is formed from a wide bandgap material, and the base 104 is formed from a narrow bandgap material. For example, among the III-V compounds, AlxGalxe2x88x92xAs (AlGaAs) may be used for the wide bandgap material, and a ternary compound such as GaAs may be used for the narrow bandgap material.
Among other things, HBTs can achieve large gain values even in cases where the base doping level is relatively high, allowing low base resistance. This attribute is particularly advantageous in high-frequency, wireless and microwave applications.
Another type of HBT, the double heterojunction bipolar transistor (DHBT) has achieved wide popularity in high-speed applications such as RF power amplifiers, high-speed digital communication circuits, and the like. A typical DHBT, as shown in FIG. 1C, incorporates a wide bandgap material for both the emitter 108 and collector 110, and another type of semiconductor, e.g., a narrow bandgap material, for the base 104. It is generally assumed that some sort of composition grading or delta-doping can be used between the junctions to smooth out bandgap conduction or valence band discontinuities. With reference to the base-emitter depletion region 120, and the collector-base depletion region 122 as illustrated in the Figures, it is also assumed that the wide bandgap material can make a transition to the narrow region at a sufficient distance away from the active junction depletion region.
While the DHBT configuration offers higher breakdown voltage and lower offset voltages as compared to SHBTs, known DHBTs are unsatisfactory in a number of respects. For example, such devices often exhibit high knee voltages. The knee voltage, referring momentarily to FIG. 4A, is the point (408) at which IC becomes substantially constant on the VCE-IC curve (transition between the linear and saturation regions).
In DHBTs, the wider bandgap in the collector is desirable as it offers higher breakdown voltagexe2x80x94a characteristic important in power amplifiers designs. As the base-emitter turn-on voltage is similar to the base-collector characteristic (both are heterojunctions), the offset voltage 407 is lower. Both the offset and series collector resistance affect the location of the knee voltage. In power amplifiers, the point at which the dynamic load line intersects the knee effectively sets the efficiency of the amplifier. Hence, a smaller knee voltage offers higher efficiency. More particularly, a low knee voltage permits high-efficiency power amplification at lower voltages.
Another important contributor to knee voltage is the collector series resistance, which in typical DHBTs is high due to the lower electron mobility in the wide bandgap material. Devices with high knee voltage tend to be less efficient, particularly at low power supply voltages. Furthermore, suitable collector materials used in DHBTs that have a higher range of breakdown voltages typically exhibit high collector on-resistance due to low mobility.
Methods are therefore needed in order to overcome these and other limitations of the prior art.
The present invention provides systems and methods which overcome the shortcomings of the prior art. In accordance with one embodiment of the present invention, a composite collector double heterojunction bipolar transistor (CCHBT) incorporates a collector comprising two layers: a wide bandgap collector region (e.g., GaAs), and a narrow bandgap collector region (e.g., InGaP).
In accordance with one aspect of the present invention, the higher electric field is supported in the region comprising high breakdown materialxe2x80x94e.g., in the wide bandgap regionxe2x80x94thereby increasing breakdown voltage.
In accordance with another aspect of the present invention, the use of heterojunctions for the base-emitter and base-collector junctions having comparable turn-on voltages results in lower offset voltage.
In accordance with yet another aspect of the present invention, the use of the wide bandgap, high breakdown material whose thickness is less than or equal to the depletion region of the collector, followed by a narrow, high-mobility material in the remaining depletion region, reduces series resistance as well as knee voltage.